1. Field of the Invention
This invention relates to a thin disk gain media for lasers and amplifiers, and more particularly to an optical system for pumping the gain media with a large numerical aperture beam.
2. Description of Related Art
As diode lasers and diode laser arrays have become more powerful, higher power diode-pumped solid-state lasers have become possible. Many schemes have been proposed to efficiently couple the pump light from multiple high power diode bars or diode bar arrays into the solid-state gain medium. It is desirable to build a high power laser that also possesses a good mode quality, and this is a challenge as the power of the laser increases. One scheme that achieves both high power and good mode quality is the thin disk laser configuration described in U.S. Pat. No. 5,553,088 by Brauch, Giesen, Voss and Wittig and in Optics Letters volume 20, page 713 (1995).
In the thin disk configuration, the gain medium is typically a disk of a few millimeters in diameter and only a few hundred microns thick. It is attached to a heat sink on the cooling surface. That same cooling surface is coated to reflect both the pump light and the laser light. Thus the thin disk laser is an end-pumped design with the pump light and the laser light being collinear. If the pump mode and the laser mode are matched in size, then the mode quality can be quite good without any loss in efficiency. This is typical of end-pumped designs and is in contrast to side-pumped configurations. If the disk is thin enough, the cooling will be 1-dimensional and the thermal gradient will also be collinear with the laser beam. Thus, the thermal lensing across the beam will be quite small. This is in contrast to most other end-pumped designs where the thermal lensing is significant and must be partially compensated by the design of the laser cavity.
The thin disk design does have added complexity however, because the pump light must be passed through the gain medium multiple times. The paper “pumping schemes for multi-kW thin disk lasers” by Erhard, Karszewski, Stewen, Giesen, Contag and Voss in Proceedings of Advanced Solid State Lasers conference 2000, OSA Trends in Optics and Photonics Series, Volume 34, page 78 teaches that: “for quasi-three-level systems like Yb:YAG also the reabsorption of the laser wavelength in the laser active medium plays an important role. Increasing the pump absorption in an end-pumped configuration by increasing the length of the laser active medium also increases the reabsorption losses for the laser wavelength.
Therefore the total efficiency is limited in such a configuration. The way to higher efficiency is the reduction of the reabsorption losses by reducing the length of the crystal and/or by reducing the doping concentration while simultaneously keeping the absorption of the pump radiation high. In an end-pumped configuration this can be achieved only by multiple passing of the pump radiation through the medium as it is demonstrated in the thin disk design.” The authors continue by showing that increasing the number of passes the pump light makes through the medium leads to higher efficiency when thinner crystals are used.
There are secondary reasons for the multiple passes of the pump light. The disks must be kept thin in order to preserve the 1-dimensional cooling. In addition, the fracture limit scales as the inverse of the thickness of the disk. Unfortunately the maximum doping level, and thus the maximum absorption, for most gain medium is limited. One of the gain media with the strongest absorption is Nd:YVO4 (Vanadate). Vanadate is a 4 level laser and thus complete absorption of the pump is optimal. Using Vanadate with a doping of 1 at. %, four passes of the pump radiation and a 400 micron thick disk are required to absorb 86% of the pump radiation. Higher Nd doping levels in Vanadate are possible, but lead to a reduced lifetime and reduced efficiency.
Recent work has focused on designs for achieving a large number of passes for the pump light. In designs utilizing 16 passes of the pump light, the light from the diode bars is typically fiber coupled into a fiber bundle with a numerical aperture (NA) of 0.1. This pump light is imaged by a mirror onto the disk. The remaining pump light is collected by another mirror and imaged back to the disk. A series of 8 mirrors is then used to create the 16 passes of the pump light through the gain media. Each of the mirrors needs to be large enough to capture the pump beam with a numerical aperture of 0.1.
In an alternative design, a large parabolic mirror is used and 8 different segments of this mirror replace the 8 separate mirrors of the previous design. Each segment of the parabolic mirror must now have an NA of 0.1. This requires either a brighter pump source (NA<0.1) or a larger high NA parabolic mirror. A brighter pump source can produce the same spot size with a lower NA or alternatively a smaller spot size with a constant NA.
Recently two stoichiometric materials that incorporate Yb in the crystal matrix have been demonstrated. The first, YbAG, is the host crystal YAG with all of the Yttrium replaced by Ytterbium. This crystal is thus Yb:YAG with 100% Yb doping. It is described in “Laser demonstration of YbAG and Materials properties of highly doped Yb:YAG” by Patel, Honea, Speth, Payne, Hutcheson and Equall in IEEE Journal of Quantum Electronics, vol. 37, page 135 (2001). In YbAG it has been demonstrated that 100% doping of the YAG with Yb can still lead to a good laser crystal without significant degradation in the lifetime. Most importantly, all of the pump light can be absorbed in a disk of less than 300 microns with just a single pass.
A second stoichiometric crystal called KYbW is based on the host KYW with all of the Yttrium again replaced by Ytterbium. It is described in “Laser operation of the new stoichiometric crystal KYb(WO4)2”, by Klopp et al., in Applied Physics B, vol. 74, page185 (2002). The calculated absorption length in KYbW is less than 20 microns.
These highly doped stoichiometric materials present several new possibilities. One is to continue to use multiple passes of the pump light and thinner disks. This will improve the cooling. The other possibility is to design simpler and less expensive systems. Higher NA pump schemes have not previously been contemplated for thin disk systems because of the challenges of using high NA mirrors with multiple pass pumping. High NA pump schemes have several advantages, however, especially with respect to reducing complexity and cost.
A first advantage of higher NA pump schemes is that less bright pump sources can be used. Higher NA pump schemes make sense with thin disk gain media, because the pump beam does not diverge within the gain media. These less bright pump sources can include diode stacks and diode arrays with fewer beam shaping optics. Typical beam shaping optics include fast axis collimating lenses on each diode bar, beam shapers that transform the beam quality in the horizontal and vertical directions to symmetrize the pump beam, and polarizing optics that allow two diode stacks of opposite polarization to be combined. Each of these beam shaping optics help preserve the brightness of the pump source, but increase the cost and complexity of the pump source.
Second, non-imaging concentrators can be used in place of imaging systems. Lens ducts or hollow funnel concentrators can be utilized. These non-imaging concentrators convert a large beam with a low NA from a diode stack into a smaller beam with a larger NA. This allows a large diode stack, typically 1 cm square, with space between the diode bars for efficient cooling, to be used. The concentrator can reduce the beam size by a factor of 4 or 5 but the NA of the beam will increase by the same factor. A hollow funnel is the preferred embodiment when relaying the pump beam to the gain media with a minimum cost is required.
Third, multiple pump sources can be incident on the thin disk gain media from different angles. Thus individual diode bars can be aimed at the pump spot on the gain media from multiple directions. Removing the heat from these separate bars is then made easier. Multiple diode stacks displaced around the disk can be used as well to increase the power. Each diode stack has its own coupler and would deliver the pump beam to the disk from a different direction.
There is a need for an improved optical system, and its methods of use, that has a thin disk gain media. There is a further need for an optical system, and its methods of use, that has a diode-pumped thin disk gain media and utilizes a high NA pumping scheme to reduce the cost and complexity.